One aspect of this invention relates generally to the removal of heavy metals from aqueous solutions and, in particular, to the removal of heavy metals from aqueous solutions by the method of coprecipitation. As used herein, the term "heavy metals" refers to non-ferrous metals and metaloids (e.g., arsenic, selenium, and antimony) which have an atomic number greater than that of calcium.
There is increasing concern over the hazards posed by the rising levels of heavy metals within the world's water supplies. Most heavy metals are toxic to some degree to all life-forms. Aqueous concentrations of as little as 0.05 ppm can have a deleterious effect on aquatic flora and fauna. In humans, toxic heavy metal poisoning can lead to severe nervous system disorders and can cause death. Even trace amounts of heavy metals within an organism's environment are potentially dangerous, because heavy metals do not decompose over time (as do most organic pollutants) and often accumulate within the organism throughout its lifetime. This accumulative effect is accentuated in succeeding species along each food chain.
As a consequence of the increasing concern over aqueous heavy metal concentration levels, industry is being required to virtually eliminate heavy metals from its aqueous wastes. For many industries, however, this requirement is very difficult to fulfill. The metal finishing industries, for example, employ a variety of processes which generate large volumes of aqueous waste material. Many of these wastes contain high concentrations of heavy metals (often as high as 10 percent), including zinc, nickel, copper, chromium, lead, cadmium, tin, gold, and silver. The combined quantity of these wastes generated daily is very large (over one billion gallons in the United States), and the number of plants employing metal finishing processes is also large (nearly 8,000 in the United States). Numerous heavy metal removal methods have been proposed for the metal finishing industries, including dilution, evaporation, alkali-precipitation, absorption, dialysis, electrodialysis, reverse osmosis, and ion exchange, but none has been found to be entirely satisfactory.
By far the most common heavy metal removal method is alkali-precipitation. In this method, a sufficient quantity of base is added to the aqueous waste solution to precipitate the desired quantity of heavy metals as insoluble metal hydroxides. However, as governmental heavy metal regulations have become stricter, the alkali precipitation method has become exceedingly costly, more difficult to use, and in some instances inappropriate.
Alkali-precipitation must be carried out at high pH (between about 9 and about 12) in order to reduce the soluble heavy metal concentrations to within acceptable limits. Additive chemical volumes can therefore be quite high. Large quantities of base are required to raise the waste solution pH to treatment conditions and to precipitate the requisite quantity of heavy metals. Large quantities of acid are often required to reduce the pH of the resulting treated effluent prior to its recycle or disposal. Additive chemical unit costs are also quite high because a costly base, such as caustic soda, must be employed. The most preferable base, aqueous ammonia (because it is less expensive and easier to handle than caustic soda), is impractical in the alkali-precipitation method. At the high solution pH levels required by the alkali-precipitation method, aqueous ammonia forms soluble complexes with many heavy metal species (especially with copper, nickel, and zinc) thereby preventing their precipitation.
Waste streams containing hexavalent chromium, a common contaminant in many metal finishing industry waste solutions, require costly pretreatment because the alkali-precipitation method is ineffective in precipitating hexavalent chromium. The pretreatment step entails reducing the hexavalent chromium to the trivalent state by reaction with a suitable reducing agent, such as sodium bisulfite, at pH levels below 3. After pretreatment, the trivalent chromium is precipitated from the solution as a hydroxide by raising the solution pH to above about 9.
Waste streams containing organic and nitrogenous complexing agents, also common contaminants in many metal finishing industry waste solutions, require a specialized and especially costly alkali-precipitation treatment. To counter the tendency of the complexing agents to solubilize heavy metals, large quantities of calcium hydroxide must be added to the waste solution. The large quantities of base necessarily raise the pH of the solution to very high levels, and make necessary the eventual use of large quantities of acid to neutralize the resulting effluent. The necessary use of calcium hydroxide also results in significantly increased operating costs because calcium hydroxide exists as a slurry at treatment conditions and is, therefore, very difficult to handle and control. Furthermore, having to use calcium hydroxide in such high concentrations results in large precipitate sludge disposal costs because abnormally large volumes of sludge are produced. This abnormal sludge production stems from (a) the fact that, in addition to the formation of heavy metal precipitates, calcium precipitates are formed as well, and (b) the fact that calcium precipitates tend to retain a large amount of water.
Various light metals, e.g., beryllium and aluminum, are also undesirable contaminants. Prior art methods for removing light metals from aqueous systems are not satisfactory. For example, although aluminum can be removed by alkaline precipitation, the resulting flocculent or precipitate (aluminum tetrahydroxide) is very hygroscopic and is difficult to settle. In addition, alkaline precipitation is ineffective in treating concentrated aluminum solutions. Furthermore, aluminum readily redissolves out of the precipitate.
With respect to beryllium, beryllium is a small, hydrated, unreactive atom that is generally only removal from a solution by methods capable of removing sodium, e.g., ion chromatography.
Cyanide is also a hazardous contaminant. Although prior art methods for reducing the cyanide concentration in aqueous systems exist, these methods generally require that the cyanide be treated separately. Accordingly, if other contaminants are present in a cyanide-containing solution, a multi-step process is required to remove all the contaminants. The use of separate steps to remove the cyanide and other contaminants increases the cost of the treatment process.
In addition, governmental regulations restrict the amount of organic contaminants that can be present in water. Exemplary organic contaminants include volatile organics, phenolics, oil and grease, and organic contaminants measured in terms of total suspended solids (TSS), biological oxygen demand (BOD), chemical oxygen demand (COD), and total organic carbon (TOC). Unfortunately, compliance with these governmental regulations is not always possible because it is difficult to remove sufficient organic contaminants with prior art organic contaminant removal methods.